// test.java => 编译(javaC) => test.class => 运行(java) [windows,linux,mac] 
// JDK(Java Development Kit)
// JDK = JRE + java的开发工具[java,javac,javadoc,javap等]
// JRE(Java Runtime Environment)
// JRE = JVM + java核心类库(类)
